Objectives of this study are to define the nature and hemostatic functions of the activities in plasma designated as antihemophilic factor (factor VIII) and the other factor that is deficient in von Willebrand's disease (vWd). The primary source for factor VIII is bovine in origin due to the increased stability and accessibility of bovine plasma factor VIII. The biochemical and biological properties of the platelet aggregating factor (bovine von Villebrand factor) and also the human von Willebrand factor are being investigated. The mechanism of thrombin enhancement of factor VIII activity and the interaction of the antibiotic ristocetin with the von Willebrand factor and various plasma factors is being further investigated as is the interaction of factor VIII and IXa to form the factor X activator.